Distributed generation (DG) refers to the scheme of generating power by a large number of small generators connected at the power distribution system. Most distributed generators hereinafter referred to as Local Generators (LG) use renewable resources such as wind, solar and small hydro for power generation. DG has recently gained a lot of momentum in the power industry due to market deregulation. One of the problems faced by DG industry is to make sure that the LGs work in a safe environment and they can disconnect from the grid if any portion of the power distribution system where the LGs are connected becomes isolated from the main power supply due to, for example, faults. The objective is to protect the LGs and neighboring customers from electrical damage because most LGs cannot regulate voltage and frequency by themselves when operating in an electrical island. The current industry practice is to disconnect all LGs immediately after the occurrence of islands. Typically, a LG should be disconnected within 300 ms to 1 second after loss of main supply (MS) e.g. supply from the main generator (MG) according to prevalent DG interconnection standards. To achieve such a goal, each LG must be equipped with an islanding detection device. The common devices used for this purpose are modified versions of under/over voltage and under/over frequency relays. Representative examples of such relays are the Rate of Change of Frequency Relay (ROCOF) and the Vector Surge Relay (VSR), which is also known as vector shift or voltage jump relay. Another example is U.S. Pat. No. 6,815,932 which presents an improved implementation of the frequency-based islanding detection concept. It is known if the generation and load have a large mismatch in a power system, the frequency of the system will change. In view of the fact that the frequency is constant when the feeder is connected to the distribution system i.e. the MG, it is possible to detect the islanding condition by checking the amount and rate of frequency change. The ROCOF and VSR relays are based on such principles. This is the simplest islanding detection technique. However, it cannot function properly or fast enough if the generation and load mismatch is small. They often result in nuisance trips of LGs as well.
In order to overcome the above problem, active schemes that require a LG to inject small signals (or disturbances) to the system have been proposed. For example, U.S. Pat. No. 5,686,766 proposes to connect a switchable resistor at the LG terminal to disturb the total power output of the LG-resistor set. The degree of resultant power fluctuation is used to detect islanding conditions. U.S. Patent Application No. 2003/0098671 A1 proposes to inject voltage or current signals that will be used to determine the impedance of the system connected to the LG. U.S. Patent Application No. 2003/0165036A1 lets the LG inject white noise to the supply system for islanding detection. U.S. Pat. Nos. 6,219,623B1, 6,810,339B2, 6,172,899B1 proposes to shift the frequency or voltage of the LG as a disturbance. The shift is controlled in such a way as to result in a positive feedback to the frequency, which will make it easier to detect islanding conditions using frequency-based concepts. All of the above active methods suffer one common drawback, which is the interaction among the signals injected by different LGs. The interactions will degrade the quality of the signals, leading to less reliable detection of islanding conditions. Another problem faced by these methods is that some of the schemes, such as the positive feedback schemes, can only be implemented on inverter-based local generators.
In addition to the above local information based islanding detection schemes, techniques that use telecommunication means to trip islanded LGs have been used in industry. With this ‘transfer-trip’ scheme, each LG is equipped with a cellular phone like receiver. The isolating breakers each has a transmitter that sends a trip signal to the LG receivers if it opens. With current telecomm technologies, there is no major technical problem to do so. The problem is the cost and complexity. Firstly, it is expensive for areas that are not covered by radio communications. Secondly each breaker needs a transmitter and there could be several of them between the LG and the substation or MS. Thirdly, some of breakers need to be reconfigured and equipped with the capability of interfacing with the signal transmitter.
Fourthly, feeder segments including their LGs could be reconnected to a different system due to the practice of feeder reconfiguration. In this case, An LG signal receiver must have the capability to decide which signal transmitters it should listen to.
In summary, as more and more distributed generators are added to utility systems, it is highly desirable to have a reliable and low cost islanding detection technique. This is especially true for the synchronous machine based local generators.
U.S. Pat. Nos. 4,340,880 and 4,658,238 disclose a method of transmitting intelligence signals over the power transmission lines by adding at intervals a signal wave form to a carrier wave form to produce a composite wave form and the added signal wave form and on the carrier wave form and the intervals there between are detected and used to aid in deciphering the information being transmitted